The principal aims of this proposal are to study integrative mechanisms underlying behavior, long-term changes in synaptic transmission and the regeneration of specific synaptic connections between individual, identified cells. The CNS of the leech provides a favorable preparation in which physiological, chemical and morphological techniques can be used to approach such problems in the animal and in ganglia in organ culture. Many cells have been identified, their functions established and the properties of their synaptic connections analysed; moreover long-term changes develop in the nervous system following lesions or the deletion of a single neuron. We now propose to examine such questions as: What changes develop at a synapse as the result of the chronic removal of presynaptic inhibition? If a cell is killed, will other neurons sprout to take its place? And if a touch cell innervating the skin is killed will other touch cells form new endings in the denervated territory? Recently we have found that single sensory cells, interneurons and motor cells can be dissected out of the adult nervous system and kept in tissue culture for several weeks. These isolated, identified cells maintain their electrical characteristics, grow processes and form chemical and electrical synapses with appropriate targets. In this simplified system we propose to analyse in detail: 1) the chemosensitivity of the subsynaptic membrane, comparing it to that of the cell body, 2) the relation between presynaptic membrane potential and transmitter release and 3) the specificity of synaptic connections. Do individual neurons form synaptic connection with certain preferred targets? With this information we shall search for novel properties that develop in cells totally isolated from the CNS. Abnormal connections may be formed and the characteristics of transmission may change. By comparing the same cells in culture, in the animal, and during regeneration one can hope to assess more directly factors influencing the formation and maintenance of synaptic connections.